The boy, who hasn't been identified, was swimming at Scott's Cove, which is an area within the Rocky Gorge Reservoir. Fire and EMS officials said the boy was heard shouting for help around 4 p.m.
After hearing his cries, a frantic water and ground search got underway which included about 40 people searching the area, including a dive team, a dog team, rescuers in boats and a helicopter.
But at about 7 p.m. the boy was found and pronounced dead at the scene.
The water temperature was approximately 50 degrees.
The Washington Suburban Sanitary Commission says swimming is not permitted in the reservoir and patrols will be increased to deter swimmers.
The Associated Press contributed to this report. Follow @WTOP on Twitter.